Body area networks (BANs) are a low-power short-range wireless technology that can be used for medical applications, such as digital band-aids and pacemakers, and for entertainment and consumer electronics applications, including heads-up displays and wireless gaming. Body area networks are being designed for use in several radio frequency bands, including 400 MHz Medical Implant Communications Service (“MICS”) band, 900 MHz and 2.4 GHz Industrial, Scientific and Medical (“ISM”) band, and 3.1-10.6 GHz Ultra Wideband (UWB).
A symbol is a representation of one or more bit values, e.g., 0s or 1s, often used in wireless communications. The particular value represented by a symbol may be determined based on when a signal is transmitted by a device and when that signal energy is received by a device that is listening for signals. Symbols may be modulated, or made to appear to a receiving device as an intended value, in a multitude of ways. A transceiver, or a device capable of transmitting and receiving symbols, must agree with another transceiver on a modulation scheme in order for the two devices to communicate. In some modulation schemes, a symbol may be thought of as positions in which a signal may be transmitted. In these “pulse position modulation” schemes, transmission of a signal in one position may represent a first value, whereas transmission in another position may represent a different value.
A refinement of this scheme uses a burst (a concatenation of pulses or chips), as opposed to a single pulse, with multiple burst positions to represent a symbol. This scheme is called “burst position modulation.” Multiple burst slots replace a single pulse position to represent a single symbol value, and the receipt of signal energy in any of the multiple burst slots for a particular bit is interpreted as a transmission of that particular symbol value. The burst slot in which a signal is transmitted to represent a particular symbol value may change from symbol to symbol. Changing, or hopping, the burst slot from one symbol to the next in a deterministic way is called time-hopping. The hopping pattern is known to both the transmitter and the receiver. Such time-hopping mitigates interference from neighboring devices and serves to make the transmitted signal more random, thereby reducing ripple in the spectrum.
Wireless transceivers may also apply scrambling to transmitted symbols. Scrambling adds randomness to a transmitted sequence, thereby spreading signal energy more evenly over a range of frequencies. Scrambling may be applied at various levels in the transceiver. For example, scrambling may be applied at the symbol level. A scrambled transmission must be descrambled by a receiving device. Consequently, transmitting and receiving devices must be synchronized with regard to the scrambling sequence applied to the transmission.